The Visitor Pattern implementation in Ruby from the Wikipedia example

body.rb

class Body < CarElement
  include Visitable # now we have the 'accept' method
end

car.rb

class Car < CarElement
  def initialize
    # Here we build an array of objects that will be visited. This can be done
    # after initialize as well; it just needs to be built before calling 'accept'.
    @elements = []
    @elements << Wheel.new("front left")
    @elements << Wheel.new("front right")
    @elements << Wheel.new("back left")
    @elements << Wheel.new("back right")
    @elements << Body.new
    @elements << Engine.new
  end

  # go through each element and 'visit' it
  def accept(visitor)
    @elements.each do |element|
      element.accept(visitor)
    end
    visitor.visit(self)
  end
end

car_element.rb

class CarElement
  # force subclasses to override the accept method
  def accept(visitor)
    raise NotImpelementedError.new
  end
end

car_element_do_visitor.rb

class CarElementDoVisitor
  def visit(subject)
    puts "Do some other visitation: %s" % subject.class.to_s
  end
end

car_element_print_visitor.rb

# Each of the following classes visit the objects, doing something different
# each time.
class CarElementPrintVisitor
  def visit(subject)
    puts "Visiting: %s" % subject.class.to_s
  end
end

engine.rb

class Engine < CarElement
  include Visitable # now we have the 'accept' method
end

implementation.rb

c = Car.new
c.accept(CarElementPrintVisitor.new) # pass an object that will be visited
c.accept(CarElementDoVisitor.new)    # same here

single_file_visitor_pattern_example.rb

class CarElement
  # force subclasses to override the accept method
  def accept(visitor)
    raise NotImpelementedError.new
  end
end

module Visitable
  # Accept a visitor object. This will be visited, meaning its 'visit' method
  # will be invoked
  def accept(visitor)
    visitor.visit(self)
  end
end

class Wheel < CarElement
  include Visitable # now we have the 'accept' method

  attr_reader :name
  def initialize(name)
    @name = name
  end
end

class Engine < CarElement
  include Visitable # now we have the 'accept' method
end

class Body < CarElement
  include Visitable # now we have the 'accept' method
end

class Car < CarElement
  def initialize
    # Here we build an array of objects that will be visited. This can be done
    # after initialize as well; it just needs to be built before calling 'accept'.
    @elements = []
    @elements << Wheel.new("front left")
    @elements << Wheel.new("front right")
    @elements << Wheel.new("back left")
    @elements << Wheel.new("back right")
    @elements << Body.new
    @elements << Engine.new
  end

  # go through each element and 'visit' it
  def accept(visitor)
    @elements.each do |element|
      element.accept(visitor)
    end
    visitor.visit(self)
  end
end

# Each of the following classes visit the objects, doing something different
# each time.
class CarElementPrintVisitor
  def visit(subject)
    puts "Visiting: %s" % subject.class.to_s
  end
end

class CarElementDoVisitor
  def visit(subject)
    puts "Do some other visitation: %s" % subject.class.to_s
  end
end

c = Car.new
c.accept(CarElementPrintVisitor.new) # pass an object that will be visited
c.accept(CarElementDoVisitor.new)    # same here

# OUTPUT
# Visiting: Wheel
# Visiting: Wheel
# Visiting: Wheel
# Visiting: Wheel
# Visiting: Body
# Visiting: Engine
# Visiting: Car
# Do some other visitation: Wheel
# Do some other visitation: Wheel
# Do some other visitation: Wheel
# Do some other visitation: Wheel
# Do some other visitation: Body
# Do some other visitation: Engine
# Do some other visitation: Car

visitable.rb

module Visitable
  # Accept a visitor object. This will be visited, meaning its 'visit' method
  # will be invoked
  def accept(visitor)
    visitor.visit(self)
  end
end

wheel.rb

class Wheel < CarElement
  include Visitable # now we have the 'accept' method

  attr_reader :name
  def initialize(name)
    @name = name
  end
end